1. Field of the Invention
This invention concerns a two-stroke cycle engine which uses a layer of scavenging air pressurizing the crankcase. More specifically, it concerns a small two-stroke cycle engine using a preceding air-layer for scavenging, which drives a layer of scavenging air in advance of the fuel-air mixture.
2. Description of the Related Art
Two-stroke cycle engines belonging to the prior art take advantage of the fact that a negative pressure is created in the crankcase when the piston reaches the top of its stroke. This negative pressure causes the fuel-air mixture to be sucked into the crankcase. When the piston reaches the bottom of its stroke, the pressurized fuel-air mixture in the crankcase reaches the scavenging port and is conducted from the crankcase into the combustion chamber. The fuel-air mixture fills the combustion chamber, pushing the exhaust gases ahead of it. In this scavenging process, the opening duration of the scavenging port and the exhaust port experiences significant overlap, with the result that approximately 30% of the fuel-air mixture is sucked out with the exhaust gases. This is the primary cause of the large component of THC (total hydrocarbons) in the exhaust, and it results in the wastage of fuel.
To reduce the quantity of fuel-air mixture which is pushed out of the combustion chamber, scavenging air designs which drive a layer of air ahead of the fuel-air mixture have been proposed. In engines which use scavenging air, the fuel-air mixture goes into the crankcase as the piston travels upward in the intake process. At the same time, air is sucked into the crankcase through a scavenger passage connected to the scavenging port so that the passage is filled with air. In the combustion and exhaust processes which occur when the piston drops and the scavenging port is open, the air in the scavenger passage is forced into the combustion chamber ahead of the fuel-air mixture to scavenge the exhaust gases from the combustion. Immediately after the scavenging air, the fuel-air mixture is admitted into the combustion chamber. This scavenging-air method reduces the quantity of fuel-air mixture which is pushed out of the combustion chamber to one third that which occurred with prior art engines.
A design for a scavenging-air two-stroke cycle engine which forces a layer of air ahead of the fuel-air mixture, in which the fuel and air valves on the carburetor are realized as a single valve, is disclosed in the Japanese Patent Publication (Kokai) 10-252565.
In the prior art scavenging-air engine which drives a layer of air ahead of the fuel-air mixture, the preceding layer of air admitted to the cylinder and crankcase through the air control valve was routed through the same number of passages (either two or three) as there were scavenging ports downstream from the air control valve. These were connected to the passages for the scavenging ports of the cylinders by rubber tubes. The air was fed through lead valves on the scavenger passages to passages on the cylinder and crankcase.
The air introduced via the air control valve was sucked into the crankcase temporarily when the cylinder of the piston was pressurized. When the piston dropped and scavenging occurred, the scavenging air was led into the combustion chamber from the scavenging port.
In another prior art design proposed in the Japanese Patent Publication (Kokai) 7-139358, an air passage was provided which fed into the scavenger passage at a location adjacent to the scavenging port. A non-return valve was provided on the air passage, as was a control valve. The control valve was interlinked with the operation of the engine throttle. In this engine, the crankcase experienced negative pressure when the piston was up. At the same time that the fuel-air mixture was sucked into the crankcase through its supply port, the non-return valve was opened and the air was sucked in through the air passage. This air would completely or partially fill the scavenger passage. When the piston fell during the ignition and exhaust processes and the scavenging port was opened, first the air would rush into the combustion chamber and then the fuel-air mixture would be supplied.
With this prior art technique, a means was devised which would supply the air from the scavenging port to scavenge the combustion chamber quickly at the start of the scavenging process so as to minimize the quantity of fuel-air mixture lost through the exhaust port. This device admitted the fuel-air mixture from the crankcase into the combustion chamber via the scavenging port with a slight delay after the scavenging air was admitted.
This sort of two-stroke cycle engine which admitted a layer of air in front of the fuel-air mixture reduced the quantity of mixture exhausted with the combustion gases, prevented an excessive quantity of THC (total hydrocarbons) from being exhausted, and minimized the quantity of fuel wasted.
In the preceding air-layer type two-stroke cycle engine proposed in the Japanese Patent Publication (Kokai) 10-252565, the preceding air was brought in through a number of rubber tubes with lead valves which was equal to the number of scavenging ports. The design thus required a large number of parts and assembly processes, both of which drove the cost up. Furthermore, the supply passages for the air were provided on the outside of the cylinder, so the dimensions of the engine in its axial direction were increased.
In a two-stroke cycle engine, combustion must be kept stable by supplying a rich mixture with little air when the engine is operating under a light load, including when it is idling, and a comparatively thin mixture when it is operating under a heavy load. This will reduce fuel consumption and decrease the harmful component of the exhaust gas. However, in the prior art design proposed in the Japanese Patent Publication (Kokai) 7-139358, the airflow supplied via the supply passages during scavenging is not controlled to conform to the operating state of the engine.
Under light load conditions, then, such as when the engine is idling, too much air is supplied; and it would be difficult to stabilize combustion by limiting the quantity of air admitted to produce a rich mixture. Similarly, it would be difficult to maintain a thin mixture under heavy load conditions in order to reduce the pollutants in the exhaust gas and lower the fuel consumption.
In the invention disclosed in the Japanese Patent Publication (Kokai) 9-125966, a mixture control valve is provided to open and close the mixture passage which connects the carburetor to the crankcase, and an air control valve is provided to open and close the air passage which connects the air cleaner. The mixture control valve and air control valve are linked so that it is possible to control the flow rate of the fuel-air mixture and that of the air in such a way that their ratio remains constant.
In this type of preceding air-layer type two-stroke cycle engine, when the engine is idling the negative pressure in the air passage increases until it is higher than that in the fuel mixture passage. This causes the throttle to open more, suddenly increasing the speed of the engine. The delay in the fuel supply allows the excessively rich fuel mixture to be thinned out by radically increasing the quantity of preceding air. The extra air reduces the concentration of the fuel mixture.
But when the engine is operating at high speed, an increase in the quantity of air will not be followed by an increased quantity of fuel. The concentration of the fuel will decrease and proper combustion will no longer be possible. Problems with acceleration or engine cut-off may result.
However, in the inventions disclosed in the Japanese Patent Publications 7-139358, 10-252565 and 9-125966, no means are provided to control the ratio of air flow to air-fuel mixture flow during normal operation so as to prevent an excessive quantity of air from being supplied when the engine suddenly accelerates as was described above.
Furthermore, when this sort of two-stroke cycle engine is used in a lawnmower, in many cases it must operate while mounted obliquely. When an obliquely mounted engine operates, fuel collects in the portion below the passage for the fuel-air mixture. When the position of the engine changes, this fuel is sucked in suddenly, resulting in combustion problems due to excess fuel in the engine.
However, the prior art techniques did not offer any method to counteract problems arising from the engine being operated while mounted obliquely.
Generally in two-stroke cycle engines, the passage for the scavenging air which leads into the chamber inside the crankcase follows a smooth curve inside the crankcase, goes through the surface where the crankcase is attached to the cylinder, and is connected to the scavenging port in the cylinder.
FIG. 39 shows an example of a scavenger passage in a two-stroke cycle engine belonging to the prior art. In this drawing, 1 is the engine, which is configured as follows.
2 is the cylinder; 5 is the crankcase. The cylinder 2 and crankcase 5 are fixed to each other with gasket 3 between them by bolts 110 at surfaces 04 and 05. 6 is the crankshaft, 7 is the cylinder head, 10 is the air passage and 60 is the center of the crankshaft.
9 is the scavenging port, which opens into the side of the cylinder 2. 109a is the scavenger passage formed in the cylinder 2, which connects to the scavenging port 9. 109c is the inlet for the scavenger passage formed in the crankcase 5, which opens into the crank chamber. 109b is the scavenger passage in the crankcase 5. It follows a smooth curve inside the crankcase 5 and connects scavenger passage 109a in the cylinder 2 with scavenging inlet 109c. 
In this two-stroke cycle engine, the scavenger passage comprises passage 109b in crankcase 5 and 109a in cylinder 2, which meet at the surfaces 04 and 05. Because scavenger passage 109b in crankcase 5 is curved, it has a portion 16 which protrudes between the upper wall of the passage 109b and surface 04.
In the Japanese Patent Publications 58-5423 and 58-5424, scavenger passages are disclosed such that a curved scavenger passage in the crankcase and a scavenger passage in the cylinder come together at the surface where the crankcase and the cylinder are fixed to each other.
However, in the configuration of the scavenger passage in the two-stroke cycle engine shown in FIG. 39, scavenger passage 109b in crankcase 5 is a curved passage with a portion 16 which protrudes between the upper wall of the passage 109b and surface 04. When the crankcase is cast, the die to form scavenger passage 109b in crankcase 5 cannot be removed as a single die in the direction of axis 61 of the cylinder.
With the prior art design shown in FIG. 39, then, to enable the die for scavenger passage 109b to be removed, several different dies were combined to cast the scavenger passage. This complicated the casting work and increased the number of casting processes. Also, because it required combining a number of dies, the probability of a defective cast due to slippage of a die increased.
In the Japanese Patent Publications (Kokai) 58-5423 and 58-5424, too, a protruding portion is formed between the upper wall of the scavenger passage and the surface where the cylinder is fixed to the crankcase. These designs, then, suffer from the same problems as were just described.
Further, with the prior art design shown in FIG. 39, the curved scavenger passage in the crankcase and the scavenger passage in the cylinder meet to form a scavenger passage which goes through the surface where the crankcase and the cylinder are fixed to each other. The fuel in the fuel-air mixture which flows through the scavenger passage will thus seep into the microscopic gap between surfaces 04 and 05 where gasket 3 is inserted. When the engine is operated in an oblique position, this fuel will return to the scavenger passage and result in defective combustion.
In the Japanese Patent Publications (Kokai) 58-5423 and 58-5424, too, a scavenger passage in the crankcase and a scavenger passage in the cylinder meet to form a common passage which goes through the surface where the crankcase and the cylinder are fixed together. These designs thus suffer from the same problem which is described above.
In a two-stroke cycle engine with a pressurized crankcase, the design takes advantage of the fact that a negative pressure is created in the crankcase when the piston reaches the top of its stroke. The fuel-air mixture is sucked into the crankcase through the air inlet. When the piston reaches the bottom of its stroke, the scavenging port opens and the pressurized fuel-air mixture in the crankcase is conducted from the crankcase into the combustion chamber via the scavenger passage and the scavenging port. The fuel-air mixture fills the combustion chamber, pushing the exhaust gases ahead of it.
In the scavenging process in a two-stroke cycle engine, the opening duration of the scavenging port and the exhaust port experiences significant overlap. To address this problem, a number of devices have been proposed to prevent the fuel-air mixture from being sucked out with the combustion gases and to insure that the mixture fills the combustion chamber uniformly.
One such device is proposed in the Japanese Utility Model Publication (Kokai) 1-44740. In this proposal, a two-stroke cycle engine has two scavenger passages, one on the right and one on the left, which lead up from the crankcase. Their upper ends curve toward the axial direction of the cylinder, and they lead into the cylinder. The angles at which the surfaces of the upper walls of the curving scavenger passages meet the cylinder vary continuously from one side to the other.
In this sort of scavenging two-stroke cycle engine with a pressurized crankcase, it is necessary to reduce the quantity of fuel-air mixture which escapes with the exhaust gases, eliminate the exhaust of a large quantity of THC (total hydrocarbons) and minimize the wastage of fuel.
In the scavenging two-stroke cycle engine with a pressurized crankcase proposed in the Japanese Utility Model Publication (Kokai) 1-44740, the angles at which the surfaces of the upper walls of the tops of the scavenger passages meet the cylinder, that is, the angles at which the air is blown into the cylinder, do vary continuously from one side of the scavenging port to the other. However, as can be seen in FIG. 3 of the same publication, the angles xcex8 of the airflow differ from each other. The portion (xcex8a) nearer the exhaust port, which is shown in FIG. 3 (a) is larger than the portion (xcex8c) nearer the intake port, which is shown in FIG. 3 (c).
Thus in the prior art design proposed in the Japanese Utility Model Publication (Kokai) 1-44740, the angle xcex8 of the airflow in the location closer to the exhaust port is made larger. As a result, after the fuel-air mixture forced into the combustion chamber from the portion of the scavenger passage closer to the exhaust port reaches the top of the combustion chamber, it is liable to be caught in the flow of combustion gases travelling toward the exhaust port. The fuel-air mixture supplied from the location closer to the exhaust port, then, is likely to escape out the exhaust port with the combustion gases which comprise the exhaust gas. This will increase the quantity of the THC (total hydrocarbons) which are exhausted and the quantity of fuel which is wasted. The scavenging efficiency will decrease, the density of the fuel-air mixture filling the combustion chamber will be lower, and the engine output will go down.
In a two-stroke cycle engine which is used in a lawnmower, the long scavenger passage which connects the scavenging port to the crankcase and supplies the air-fuel mixture from the crankcase to the combustion chamber must be formed in both the crankcase and the cylinder. The crankcase and cylinder, which are generally made of cast aluminum, must assume a complicated shape, so that their casting requires many processes.
Two-stroke cycle engines for universal applications have been proposed in the Japanese Patent Publication (Kokai) 58-5424 and the Japanese Utility Model Publication (Kokai) 4-26657, among others.
In the Japanese Patent Publication (Kokai) 58-5424, the crankcase has both a primary and an auxiliary scavenging port. The two scavenger passages which lead into the interior of the crankcase, i.e., into the crank chamber, go from the interior of the crankcase through the surface where the crankcase and the cylinder are fixed together. In the cylinder, these two scavenger passages connect to the primary and auxiliary scavenging ports.
In the Japanese Utility Model Publication (Kokai) 4-26657, two pairs of curved scavenger passages go from the interior of the crankcase through the surface where the crankcase and the cylinder are fixed together and then through the interior of the cylinder.
As has been discussed, this is an air-layer type scavenging two-stroke cycle engine. In it, a long scavenger passage which connects the scavenging port and the crankcase is formed in the interior portions of the crankcase and the cylinder. In addition, an air passage is formed in the cylinder to transport the preceding air to the scavenging port. This air passage connects to some intermediate location on the scavenger passage. The crankcase and cylinder, which are generally made of cast aluminum, must be shaped in such a way that the scavenger passage forms a smooth channel in order to minimize the resistance experienced by the fuel-air mixture and the airflow. The shapes of the dies for the casting must be simple, the number of dies must be small, and the engine must be able to be produced in a small number of production processes.
In the design in the Japanese Patent Publication (Kokai) 58-5424, however, the inventors limited their improvement to giving the two long primary and auxiliary scavenger passages in the crankcase and cylinder a smooth contour and so reducing the resistance of the two channels. They did not devote any attention to improving the shape or number of dies used when the crankcase and cylinder were cast or to improving the casting work by reducing the number of casting processes required.
In the design proposed in the Japanese Utility Model Publication (Kokai) 4-26657, the inventors limited their improvement to providing a shape for a curved scavenger passage running from the interior of the crankcase through the interior of the cylinder and preventing the fuel-air mixture from escaping toward the exhaust side. In this prior art design, then, just as with that discussed above, no consideration was given to the casting of the crankcase and cylinder.
In a two-stroke cycle scavenging engine using a layer of air, there is a scavenger passage which connects the scavenging port on the side of the cylinder and the crankcase; an air passage which connects to the scavenger passage at a point midway along its length and supplies scavenging air from the air cleaner to the scavenger passage; and a passage which supplies the fuel-air mixture produced in the carburetor to the crankcase. Before the fuel-air mixture is supplied from the scavenging port to the combustion chamber, a mass of preceding air filtered by the air cleaner is conducted into the combustion chamber by way of the air passage, scavenger passage and scavenging port. This air scavenges the chamber, enhancing both the scavenging and the combustion efficiency.
Two inventions which have proposed such two-stroke cycle scavenging engines which use a layer of air are those disclosed in the Japanese Patent Publications (Kokai) 9-125966 and 10-252565.
In the invention disclosed in the Japanese Patent Publication (Kokai) 9-125966, there are two air cleaners. The outlet of one of the air cleaners connects via the carburetor to the supply passage for the fuel-air mixture. The outlet of the other air cleaner connects through a control valve to the passage which supplies the preceding air to the scavenger passage.
In the invention disclosed in the Japanese Patent Publication (Kokai) 10-252565, the passage which supplies the fuel-air mixture from the carburetor to the crankcase is parallel to the passage which supplies the preceding air to the scavenger passage. The air inlet of the carburetor and the inlet of the air supply passage are connected directly to the outlet of the air cleaner.
When this air layer-type scavenging two-stroke cycle engine is started up, the quantity of air which is to go through the air supply passage must be controlled and a negative pressure must be obtained. The quantity of fuel-air mixture supplied to the combustion chamber from the air mixture supply passage by way of the crankcase and scavenger passage must be increased to produce a rich mixture and so improve the starting characteristics of the engine.
In the invention proposed in the Japanese Patent Publication (Kokai) 9-125966, the air control valve is shut during start-up, so the air supply passage is closed. The air passage on the carburetor is open, and the fuel-air mixture supplied from the carburetor to the air mixture supply passage is increased to produce a rich mixture. However, this design requires two air cleaners, so the configuration is complicated and large, which drives up the equipment cost.
In the invention proposed in the Japanese Patent Publication (Kokai) 10-252565, the air inlet of the carburetor and the inlet of the air supply passage are connected directly to the outlet of the air cleaner. For this reason it would be extremely difficult to completely shut off the preceding air, that is, the air which flows into the air supply passage, at the outlet stage of the air cleaner during start-up.
It is thus impossible, in this invention, to supply a rich mixture to the combustion chamber. And since it is difficult to achieve a high negative pressure as well, the start-up characteristics are unavoidably poor.
Furthermore, in the two prior art designs discussed above, the choke valve which closes off the air passage when the engine is started up is generally a rocking choke valve which rotates about its valve shaft. When the engine is connected to a lawnmower or other working machine and is to be operated, the operating lever of the choke valve gets in the way when the engine is started with the recoil starter. Also, the rocking diameter of the operating lever is considerable, which makes it difficult to operate and so affects the ease of operation.
The present invention was developed to address the problems associated with the prior art.
The first objective of this invention is to simplify the configuration of the scavenger passage in a scavenging two-stroke cycle engine using a layer of air, to make the engine smaller and lighter, to reduce the number of parts and processes required to produce the engine, and to reduce the production cost.
The second objective of this invention is to provide an air-layer-type scavenging two-stroke cycle engine such that the quantity of fuel-air mixture exhausted with the combustion gases is reduced, and at the same time the quantity of air supplied to the combustion chamber through the scavenging port is controlled via a cut-off valve so that it remains in proper proportion to the quantity of fuel-air mixture. In such an engine, an appropriate concentration of fuel-air mixture is maintained through the entire operating range of the engine. Combustion remains constant when the engine is operated under a light load, and the rate of fuel consumption and the proportion of pollutants in the exhaust are reduced when the engine is operated under a heavy load.
The third objective of this invention is, in an air-layer-type scavenging two-stroke cycle engine, to prevent the concentration of the fuel mixture from becoming too thin due to an excessive intake of air when the engine accelerates suddenly and so maintain proper combustion, thus preventing the engine from hesitating or cutting out.
The fourth objective of this invention is to prevent fuel from collecting at the bottom of the fuel passage when the engine is installed obliquely, and so prevent the defective combustion which would result from an excess of fuel.
The fifth objective of this invention is to make it possible to cast the scavenger passage for a two-stroke cycle engine in such a way that a single die could be used and the piece could be removed from the die in the axial direction of the cylinder. The casting process would be simplified and the number of sub-processes reduced. This would prevent defects resulting from the dies slipping. It would also prevent the imperfect combustion which occurs when the engine is mounted obliquely due to fuel flowing back into the scavenger passage from the surface where the cylinder and crankcase are fixed together.
The sixth objective of this invention is, in a two-stroke cycle engine using pressurized air to scavenge the crankcase, to prevent the fuel-air mixture from escaping out the exhaust port, the reduce the quantity of THC (total hydrocarbons) in the exhaust, to improve the efficiency of the scavenging, to increase the concentration of the fuel-air mixture in the combustion chamber, and to improve the combustion and so enhance the output of the engine.
The seventh objective of this invention is to provide a two-stroke cycle engine with a scavenger passage in the crankcase and cylinder and a method of casting such an engine such that a smooth scavenger passage with little flow resistance could be formed, simply-shaped dies could be used to cast the engine, the number of dies could be reduced, and the number of casting processes could be reduced.
The eighth objective of this invention is to provide a two-stroke cycle engine with a simple, compact, low-cost configuration which would be able to supply a rich fuel-air mixture to the combustion chamber during start-up, and which would enjoy improved start-up characteristics as a result of achieving a high negative pressure.
The ninth objective of this invention is to provide a valve such as a choke valve with a simple and compact configuration to improve the operating characteristics of the engine.
This invention, then, comprises an exhaust port on the side of the cylinder; a scavenging port on the side of the cylinder; a fuel passage, which supplies fuel-air mixture to the crank chamber through the intake port on the side of the cylinder during the time of the elevation of the piston; an air passage, which supplies scavenging air from the air cleaner towards the inner side of the engine; an insulator, in which the fuel passage and the air passage run in parallel; a non-return valve, which is provided on the insulator facing towards the inner side of the engine, to allow the scavenging air to flow only towards the inner side of the engine; a pair of branching air passages to connect an air supply chamber provided at the inner side of the non-return valve and a branching scavenger passage opened to the scavenging port, which are provided within the wall of the cylinder: and a pair of scavenger passages, one end of which is connected to the scavenging port, and another outlet end of which is opened to the crank chamber, the pair of scavenger passages are provided within the wall of the crankcase.
With this invention, scavenger passages are formed in halves, with one half of each inside the walls of the crankcase and the other half inside the walls of the cylinder. This results in long scavenger passages which, when filled with air, can scavenge the crankcase ahead of the fuel-air mixture. Since the mixture is supplied after the crankcase is completely scavenged, the quantity of mixture lost through the exhaust port can be minimized.
Since all the scavenging and air passages are formed inside the crankcase and cylinder, no external pipes or mounting hardware are needed to create air passages. Both the parts count and the number of assembly processes are therefore reduced.
Ideally, as in claim 2, in addition to the configuration disclosed in claim 1, the end surface of the outlet end of the scavenger passage in the crankcase forms right angles with respect to the axis of the crankshaft, and a microscopic gap is created between the end surface of the outlet and the end surfaces of the crank webs which are perpendicular to the crankshaft, which constitutes disk valves, as the opening area of the outlet of the scavenger passage varies as the crank webs rotate.
As in claim 3, the opening area of the outlet of the scavenger passage is formed so that the opening area opens more with the rotation of the crank webs as the opening area uncovered by the crank web grows larger.
If configured as described above, the area of the outlets of the scavenger passages will increase with the rotation of the crank webs. In this way the velocity of the scavenging air which flows from the outlets through the scavenger passages and from the scavenging ports into the combustion chamber can be controlled. This allows us to reduce the quantity of fuel-air mixture which is dragged into the exhaust gas stream, and so minimize the loss of fuel-air mixture.
Ideally, the branching air passages and the branching scavenger passages formed on either side of the cylinder are surrounded by virtually parallel walls which run in the same direction.
If configured in this way, the walls of the branching scavenging and air passages can be formed integrally to and virtually parallel with the cylinder. This allows the cylinder to be cast using a single sliding die. This simplifies the configuration of the die and reduces the cost of producing it.
The invention disclosed comprises a two-stroke cycle engine using a preceding air-layer for scavenging with an exhaust port on the side of the cylinder; a scavenging port on the side of the cylinder; an intake port on the side of the cylinder fuel passage, which supplies fuel-air mixture through a mixture control valve on the carburetor to the crank chamber during the time of the elevation of the piston; a scavenger passage opened to the scavenging port; an air supply port, which supplies scavenging air from the air cleaner to the scavenger passage; a cam which is interlocked with the mixture control valve; a cam follower which engages with the cam; and an air control valve in the upstream of an air passage which controls the diameter of the air passage, and the air control valve being operated by the cam and the cam follower in such a way as to supply a quantity of scavenging air proportional to the quantity of fuel-air mixture determined by the opening of the cam and the mixture control valve to control the fuel-air mixture.
Ideally, in addition to the configuration the air control valve comprises a valve seat midway along the air passage and an umbrella-type valve which can be attached to or removed from the valve seat and which opens and closes the air passage, the cam is fixed to the rotary shaft of the mixture control valve, the cam is configured with an inner cam which is formed on the inside of the edge at a given height raised up on the outer side along the circumference so that, if a spring exerts force in the direction which closes the air control valve, when the edge of the inner cam engages with the cam follower, the operation of the mixture control valve for the fuel-air mixture is transmitted to the air control valve, and the operation opens the air control valve against the force of the spring.
Ideally, like the means, which is the second preferred embodiment of the internal cam in the air control valve has, in addition to the configuration a valve seat midway along the air passage and an umbrella-type valve which can be attached to or removed from the valve seat and which opens and closes the air passage, the cam is fixed to the rotary shaft of the mixture control valve, the cam is configured with an inner cam which is formed on the inside of the edge at a given height dropped down on the outer side along the circumference so that, if a spring exerts force in the direction which closes the air control valve, when the edge of the inner cam engages with the cam follower, the operation of the mixture control valve for the fuel-air mixture is transmitted to the air control valve, and the operation opens the air control valve against the force of the spring.
If configured in this way, the air control valve is interlocked with the valve which controls the flow rate of the fuel-air mixture. Thus when the engine operates under a light load, the air control valve is closed more, and the quantity of air is reduced. This allows stable combustion with a rich mixture. Under heavy load conditions, the air control valve is opened more, and the engine operates with a thin mixture. In this way we can prevent noxious substances with high THC (total hydrocarbons) from being exhausted.
If configured as described above, the opening ratio of the throttle valve and air control valve can easily be controlled in response to a change in the angular position of the mixture control valve.
Ideally, in addition to the configuration, an air passage which connects to the outlet of the supply passage for the fuel-air mixture runs inside the insulator. This insulator is attached to the side of the cylinder downstream from the outlet of the supply passage and the air control valve and runs in the same direction as the air supply outlet. The air passage runs in the same direction as the insulator.
The invention of this application comprises a two-stroke cycle engine using a preceding air-layer for scavenging with an exhaust port on the side of the cylinder; a scavenging port on the side of the cylinder; a fuel passage, which supplies fuel-air mixture to the crank chamber through the intake port on the side of the cylinder during the time of the elevation of the piston; a scavenger passage to be connected to the scavenging port; an air passage, which supplies scavenging air from the air cleaner toward the inner side of the engine; an insulator, in which the fuel passage and the air passage run in parallel; a non-return valve, which is provided on the insulator facing toward the inner side of the engine, to open or close the air passage by means of the negative pressure in the scavenger passage; and a connecting passage with a small diameter to link the air passage and the fuel passages so that negative pressure in the air passage forces the fuel-air mixture in the fuel passage into the air passage.
With the invention of this application, the fuel-air mixture in the fuel passage is supplied to the air passage through a small-diameter connecting passage when the negative pressure in the air passage becomes greater than that in the fuel passage because the engine is idling. Thus when there is an excessive quantity of air in the air passage during sudden acceleration, fuel-air mixture from the fuel passage will be introduced and mixed with that air.
By forcing fuel-air mixture into the airflow in the air passage, we prevent the new air supplied to the cylinder from the scavenging port from creating a mix with too much air. This method prevents the fuel mixture from becoming too thin during sudden acceleration of the engine and so improves the acceleration characteristics.
Furthermore, since the throttle opens during high-speed operation, the pressure differential between the fuel and air passages virtually disappears. Thus virtually no fuel-air mixture will flow from the fuel passage through the small-diameter connecting passage and into the air passage at high speeds. The method prevents fuel-air mixture from getting into the preceding air and so helps maintain the required exhaust characteristics.
When the engine is mounted obliquely, the fuel-air mixture in the fuel passage can flow through the small-diameter connecting passage into the air passage. It will thus not collect in the bottom of the fuel passage, so that a large quantity of fuel is suddenly sucked into the cylinder when the engine""s orientation is changed. This design, then, will prevent imperfect combustion.
With the invention, then, we can achieve an air-layer-type scavenging two-stroke cycle engine which produces the effect through a very simple device, i.e., a small-diameter connecting passage between the air and fuel passages which causes the negative pressure of the air passage to draw the fuel-air mixture in the fuel passage into the air passage. This obviates the need for a complicated control device.
In invention the small-diameter connecting passage in the configuration connects the air and fuel passages at a point downstream from the non-return valve.
If configured in this way, the connecting passage goes into the air passage downstream from the non-return valve. When the engine is mounted obliquely, when it is, say, rotated 180xc2x0, the fuel which collects in the lower end of the fuel passage will flow through the connecting passage into the air passage. This will prevent too much fuel from flowing into the combustion chamber and affecting the rate of combustion, improving both the engine""s operation and its effect.
In invention the small-diameter connecting passage in the configuration can connect the air and fuel passages at a point upstream from the non-return valve.
If configured in this way, the connecting passage goes into the air passage upstream from the non-return valve. The diameter of the mouth of the connecting passage can be increased without producing a drop in engine output. This further improves the acceleration characteristics during sudden acceleration.
The following embodiments are preferred for the devices.
(1) The connecting passage is formed in the insulator gasket interposed between the mounting surfaces of the insulator and cylinder, or in the carburetor gasket interposed between the mounting surfaces of the carburetor and the insulator. If one of these configurations is adopted, it will be possible to fashion a connecting passage merely by creating a slit of the same diameter as the connecting passage in either the insulator gasket or the carburetor gasket. Creating a connecting passage in this way is straightforward and does not require a large number of processes.
(2) The connecting passage is cut into the surface of the insulator where it is mounted to the cylinder.
(3) The connecting passage is cut into the surface of the cylinder where it is mounted to the insulator.
(4) The connecting passage is formed in the carburetor gasket which goes between the carburetor and the insulator.
(5) The connecting passage is cut into the surface of the insulator where it is mounted to the carburetor.
(6) The connecting passage is cut into the surface of the carburetor where it is mounted to the insulator.
(7) The connecting passage comprises a small hole in either the insulator, the carburetor, or the cylinder, which connects the air passage to the fuel passage.
(8) There is a non-return valve on the small hole which permits flow only in the direction from the fuel passage to the air passage.
(9) The connecting passage is placed so that one end connects the air and fuel passages downstream from the non-return valve while the other end connects them upstream from the valve.
If the connecting passage is configured in this way, with one end between the air and fuel passages downstream from the non-return valve, the fuel-air mixture in the fuel passage can flow through the connecting passage and into the air passage when the engine is mounted obliquely. This prevents fuel from collecting at the bottom of the fuel passage and being sucked into the cylinder suddenly when the position of the engine changes, so it eliminates imperfect combustion due to an excess of fuel. The other end of the connecting passage connects the air and fuel passages upstream from the non-return valve. This makes it possible to increase the diameter of the outlet of the connecting passage without a drop in engine output, thus improving the engine""s ability to accelerate suddenly.
The invention of this application comprises A two-stroke cycle engine with a scavenger passage which connects a scavenging port on the side of the cylinder to the crank chamber inside the crankcase, and goes through the mounting surface where the cylinder and crankcase are attached to each other; and a removable guide with a surface forming a curved smooth channel which is attachable to the scavenger passage in the crankcase from the mounting surface, and forms a portion of the scavenger passage with the curved channel.
The guide has a positioning tooth which engages with the hole in the gasket for the surface where the cylinder and crankcase are attached to each other.
The guide is fixed to the crankcase when its tooth engages in an indentation in the crankcase.
If the engine is configured as described above, the fuel-air mixture from the crank chamber in the crankcase is led into a scavenger passage one portion of which comprises a guide with a smoothly curved channel. The mixture flows through the scavenger passage formed as a smooth channel and is supplied to the scavenging port. Because the scavenger passage is a smoothly curved channel without any right angles, the fuel-air mixture flows smoothly and rapidly without any flow loss such as a decrease in flow velocity as it is supplied to the scavenging port.
The guide is mounted on the crankcase in such a way that it can be removed by pulling it away from the surface where the crankcase is attached to the cylinder along the axial direction of the cylinder. This obviates the need for a tooth between the upper wall of the scavenger passage and the surface where the crankcase is attached, as was required in the prior art. The guide performs the same function as the tooth. When the crankcase is cast, even if a single die is used to form the scavenger passage inside the crankcase, the die can easily be removed in the axial direction of the cylinder.
This design simplifies the casting procedure by which the scavenger passage is formed, and it reduces the number of casting processes required. Because the scavenger passage can be formed using a single die, there is no possibility that one of several dies will slip out of position, as sometimes happened with prior art techniques, and ruin the casting. This design, then, improves the quality of the crankcase containing the scavenger passage.
The guide has a positioning tooth which engages in a hole in the gasket between the cylinder and crankcase. When the tooth engages in the hole in the gasket, its position is assured, and the position of the gasket is also assured. The fact that the guide has been inserted can be ascertained by how the gasket is positioned, so there is no chance that the guide will be forgotten.
The crankcase has an indentation which serves as the slot into which the tooth on the guide engages. Because the guide is fixed to the crankcase, it will always be positioned correctly. The surface of the channel on the guide connects smoothly to the scavenger passage in the crankcase.
Ideally, The guide should have a depression in the surface at which it is fixed to the crankcase.
If the engine is configured in this way, the slight gap between the crankcase and cylinder where the gasket is interposed will be smaller. Less fuel will seep into the gap from the fuel-air mixture flowing through the scavenger passage connecting the crankcase and the cylinder through the common surface where they are attached to each other, which comprises the scavenger passage formed by the crankcase and the guide, and the scavenger passage in the cylinder. The fuel flows downward through the depression formed in the guide, so even if the engine is mounted obliquely, the fuel cannot return to the scavenger passage. This eliminates imperfect combustion due to fuel flowing back into the scavenger passage.
Ideally, The guide is distinguished by the fact that it is painted on.
If the guide is configured in this way, by being painted on, it will always be perfectly plain whether or not it is there. This will eliminate the possibility that it will be forgotten during assembly. By using a different color for each type of machine, we can simplify our parts control.
A preferred embodiment of this application is as follows.
The guide is formed by molding a deep-drawing sheet as a single piece with the gasket between the cylinder and crankcase. If produced in this way, the guide and the gasket are one piece, and deep drawing allows the channel surface and the depression to be formed on the front and reverse sides of the sheet at the same time. This reduces the parts count and the number of assembly processes.
The invention of this application comprises a two-stroke cycle engine with an exhaust port on the sidewall of the cylinder, which opens into the cylinder; a scavenging port on the sidewall of the cylinder positioned a slight distance apart in the circumferential direction from the exhaust port, which also opens into the cylinder; an intake port, which opens to supply fuel-air mixture to the crankcase according to the action of the piston; and a scavenger passage, which connects the crankcase and the scavenging port; wherein a blow-up angle (xcex1) of the scavenger passage, which is defined by an angle between the upper wall which connects to the scavenging port and a perpendicular line to the axis of the cylinder, varies along the circumferential direction of the cylinder, and if the blow-up angle in a location nearer the exhaust port is defined as (xcex11) and the blow-up angle in a location nearer the intake port is defined as (xcex12), then xcex11 less than xcex12.
Ideally, the blow-up angle (xcex1) of the scavenger passage varies continuously from a location nearer the intake port to one nearer the exhaust port.
Also ideally, the surface of the upper wall of the scavenger passage is formed so that it varies in one or more steps from angle (xcex12) at a location nearer the intake port to angle (xcex11) at a location nearer the exhaust port.
If the scavenger passage is configured as described above, when the action of the piston causes the exhaust port and then the slightly lower scavenging port to open, the fuel-air mixture forced into the scavenger passage from the crankcase will flow from the scavenging port into the combustion chamber.
Because the blow-up angle of the upper wall connecting the scavenger passage to the scavenging port is greater at a location nearer the intake port than it is at a location nearer the exhaust port, the fuel mixture which enters the chamber from the location nearer the exhaust port will flow along the top of the piston at a high speed without being dispersed. This will prevent it from getting caught in the exhaust gas stream and so reduce the quantity of fuel lost through the exhaust port. The fuel-air mixture which enters the chamber from the location nearer the intake port will be flowing at a lower velocity than that nearer the exhaust port. It will be sent into the area around the spark plug in the upper part of the chamber, where it will be efficiently ignited and combusted.
Thus this configuration prevents the fuel-air mixture from escaping unburned through the exhaust port, improves the scavenging efficiency, and increases the concentration of the fuel-air mixture which fills the combustion chamber. This improves the combustion and increases the output of the engine. The fact that the fuel-air mixture is prevented from escaping through the exhaust port translates into a lower THC level (total hydrocarbons) in the exhaust.
The surface of the upper wall of the scavenger passage is formed so that it varies in step fashion from a large angle (xcex12) at a location nearer the intake port to a smaller angle (xcex11) at a location nearer the exhaust port. When the cylinder is cast, two dies can be used with two different blow-up angles, as described above, with the angles changing at the border between the dies. This will make it easy to remove the work from the dies and will reduce the number of processes necessary to produce the cylinder. Also, using dies with two different blow-up angles to form the scavenger passage is an easy and reliable way to control the blow-up angle.
The invention of this application comprises a two-stroke cycle engine with a scavenging port on the side of the cylinder, which opens into the cylinder; and a scavenger passage, which connects the crank chamber in a crankcase and the scavenging port, and supplies the fuel-air mixture in the crank chamber to the scavenging port; wherein the crankcase is configured in such a way that the front and rear portions, which are separated by a block at a right angle to the crankshaft which entails the axis of the cylinder, are fixed to each other by mounting hardware, a scavenger passage is provided inside both the front and rear portions of the crankcase, and the cylinder, whose scavenger passage connects to the scavenger passage in the crankcase, is fixed by mounting hardware to the mounting surface on the top of the crankcase in such a way that the scavenger passage runs through the mounting surface.
The invention of this application comprises a two-stroke engine with a scavenger passage which connects the crankcase and the scavenging port on the side of the cylinder, which opens into the cylinder and supplies the fuel-air mixture in the crankcase to the scavenging port. This two-stroke engine is distinguished by the following. In addition to having scavenger passages in both the crankcase and the cylinder, the front and rear portions of the crankcase, separated by a block at a right angle to the crankshaft, which entails the axis of the cylinder, are fixed to each other at the block by mounting hardware to form a unitary crankcase. The cylinder, whose scavenger passage connects to that in the crankcase, is fixed by mounting hardware to the mounting surface at the top of the crankcase.
Ideally, the air passage which supplies air from the air cleaner to the scavenger passage is formed inside the cylinder. The air passage connects to the middle portion of the scavenger passage inside the cylinder.
If the engine is configured in this way, the crankcase is separated by a block at a right angle to the crankshaft, and so to the axis of the cylinder, into front and rear portions, each of which has a scavenger passage inside it. The front and rear portions of the crankcase are fixed together by mounting hardware. The cylinder, which also has a scavenger passage inside it, is fixed to the upper surface of the crankcase by mounting hardware. The scavenger passage in the cylinder is thus linked to that in the crankcase, forming a long scavenger passage running through both cylinder and crankcase. Both the crankcase and the cylinder are thus compact structures with no bulges, and the scavenger passage has a gradually curved contour without angularities.
Furthermore, a crankcase with a scavenger passage running through its interior can be cast in two pieces which form the front and rear portions of the engine. These can be removed from the die at the surface where the front and rear pieces are separated and at the surface where the cylinder is mounted to the crankcase, which is perpendicular to the surface between the pieces. This allows the die to have a simple shape, simplifies removing the work from the die, and allows the engine to be forming a long scavenger passage running through both cylinder and crankcase. Both the crankcase and the cylinder are thus compact structures with no bulges, and the scavenger passage has a gradually curved contour without angularities.
Furthermore, a crankcase with a scavenger passage running through its interior can be cast in two pieces which form the front and rear portions of the engine. These can be removed from the die at the surface where the front and rear pieces are separated and at the surface where the cylinder is mounted to the crankcase, which is perpendicular to the surface between the pieces. This allows the die to have a simple shape, simplifies removing the work from the die, and allows the engine to form a long scavenger passage running through both cylinder and crankcase. Both the crankcase and the cylinder are thus compact structures with no bulges, and the scavenger passage has a gradually curved contour without angularities.
Furthermore, a crankcase with a scavenger passage running through its interior can be cast in two pieces which form the front and rear portions of the engine. These can be removed from the die at the surface where the front and rear pieces are separated and at the surface where the cylinder is mounted to the crankcase, which is perpendicular to the surface between the pieces. This allows the die to have a simple shape, simplifies removing the work from the die, and allows the engine to be cast using fewer dies. The casting procedure is simplified and requires fewer processes.
Ideally, there should be two scavenging ports along the circumference of the cylinder. There should also be two scavenger passages running from the outlets in the crankcase to the scavenging ports. These passages should run through the block separating the halves of the crankcase, and they should be arranged symmetrically along the front-to-rear dimension of the engine.
If the engine is configured in this way, the scavenger passages will run the entire length from the outlets of the crankcase to the scavenging ports in the cylinder and through the separator block, and they will be symmetrical. Thus a common die can be used to cast the front and rear portions of the scavenger passages, allowing the engine to be produced with only a few dies. The shapes of the two passages will be identical, so the cylinder will be scavenged uniformly along its circumference and filled uniformly with the fuel-air mixture.
The following embodiments are preferred for the devices of this application.
(1) The air passage branches into two passages at the inlet to the cylinder. Each of these branching air passages extends from the branch along the length of the engine, and the two run symmetrically through the block separating the halves of the crankcase. They are connected to the scavenger passages.
If the engine is configured in this way, the two branching air passages run in parallel through the block separating the halves of the crankcase. Since they are identical, a common die can be used to cast each of the passages, and fewer dies need be used overall. Since the shapes of the two air passages are identical, the action of the scavenging air will be uniform along the circumference of the cylinder.
(2) The scavenger passages and branching air passages formed in the cylinder are enclosed by walls which run virtually parallel to each other in the same direction.
If configured in this way, the walls of the scavenging and branching air passages are integral to the cylinder and run virtually parallel to each other. The sliding die for the cylinder can thus be a single piece, which simplifies the configuration of the die.
The invention of this application comprises a two-stroke engine with a scavenger passage which connects the crankcase and the scavenging port on the side of the cylinder; an air passage connected to the midpoint of the scavenger passage, which supplies scavenging air from the air cleaner to the scavenger passage; and a fuel passage, which supplies the fuel-air mixture produced in the carburetor to the crankcase. This two-stroke engine is distinguished by the following. The air cleaner has two air passages running from it in parallel, one of which is connected to the air passage, and the other of which is connected to the air inlet of the carburetor to provide air for the fuel-air mixture. A choke valve on the air cleaner opens and closes both of these air passages.
Ideally, the choke valve comprises a rotary valve which, when rotated, opens or closes the openings of the two air passages, and a knob by which the valve can be rotated.
Also, as is disclosed the choke of the choke valve engages with the case of the air cleaner in such a way that it is free to rotate. Its front surface comprises a sheet which covers or uncovers the inlets of the two air passages. A sealing ring consisting of an elastic material presses the flat surface of the valve against the openings of the inlets by elastic force and forms a fluid seal around the valve shaft with respect to the interior of the case.
A tapered protrusion is formed on the inner surface of the case of the air cleaner. When the rotary knob of the choke valve strikes the protrusion, the flat surface of the valve is pressed against the opening of either the first or the second of the two air passages.
If the choke is configured in this way, when the knob of the choke valve is turned to start up the engine, the second air passage, which connects the air cleaner to the air inlet of the carburetor, is completely closed, and only the choke hole is open. The first air passage, which connects the air cleaner to the air passage supplying the preceding air, is also completely closed when the engine is started up.
When the choke is adjusted in this way, the air from the air cleaner which is supplied to the carburetor via the second air passage is constricted by the choke hole. The fuel-air mixture produced in the carburetor is supplied via the crankcase, scavenger passage and scavenging port to the combustion chamber.
Since the first air passage from the air cleaner is completely closed by the choke at this time, the fuel-air mixture from the carburetor is supplied to the combustion chamber without any preceding air. Thus the chamber is filled with a rich fuel-air mixture. This improves the start-up characteristics of the engine.
The flat surface of the rotary choke of the choke valve covers and uncovers the openings of the two air passages. The elastic force of the O-ring or other sealing ring which is inserted around the shaft of the choke causes the flat surface to press against the openings. The choke valve thus completely and reliably closes the opening of the passage for the preceding air. This promotes the production of a rich fuel-air mixture as described above and allows a high negative pressure to be maintained.
This invention provides both a sealing ring as described above and a protrusion on the outside of the case of the air cleaner to serve as a stop for the choke, and rotating the choke valve from its initial position to its normal operating position will switch between the two aforesaid air passages. When the choke valve is rotated, its knob easily goes over the protrusion against the force of the sealing ring. The moderate friction improves the operating feel of the choke valve. When the choke valve is released, the elastic force of the sealing ring and the force of the protruding stop automatically hold the choke valve in place on the flat portion of the case in such a way that it cannot go back. This insures easy operation.
There is, as disclosed, a tapered protrusion on the inner surface of the case of the air cleaner. When the choke valve is rotated toward starting position, the knob of the choke valve goes over the protrusion. When this occurs, the elastic force generated by the deflection of the valve causes its flat surface to push down on the opening of the passage for the preceding air. This improves the sealing function of the flat surface, and thus improves the start-up characteristics.
The invention disclosed of this application is a two-stroke engine which is distinguished by the following. It has a rotary valve installed on the case in such a way that it is free to rotate which, when rotated, opens and closes the two air passages; and a rotary knob which operates the valve. The front surface of the valve comprises a sheet which covers or uncovers the inlets of the two air passages. A sealing ring consisting of an elastic material presses the flat surface of the valve against the openings of the inlets by elastic force and forms a fluid seal around the valve shaft with respect to the interior of the case.
A rotary valve configured in this way is not limited in its application to use as a choke valve for the air cleaner of a two-stroke engine. It can be used in a wide range of applications which require switching between two fluid passages by operating a rotary valve.
To summarize, the effects of the invention disclosed above are as follows.
(1) With the inventions disclosed in this application, all scavenger passages and air passages are formed inside the crankcase and the cylinder. This obviates the need for external pipes to serve as air passages as well as their mounting hardware. Fewer parts and assembly processes are required, and the engine can be made lighter and smaller.
The scavenger passage is formed of two passages, one created by walls in the crankcase and the other by similar walls in the cylinder. The result is a long scavenger passage which can be filled with air for the scavenging operation. Since the fuel-air mixture is supplied only after the crankcase has been thoroughly scavenged by this air, no fuel is lost through the exhaust port, and fuel wastage is minimized.
If configured as the outlet of the scavenger passage on the side of the crankcase opens into the crank chamber. The gap between the side of the crankcase and the crank web is reduced, and a disk valve is formed by the outlet on the crankcase and the crank web. This controls the velocity of the air forced in through the scavenging port and reduces the quantity of fuel-air mixture which becomes trapped in the exhaust gas stream.
If configured as the branching air passage and the branching passage to the scavenging port in the cylinder are both enclosed by walls that run parallel in the same direction. This allows the sliding die for the scavenger passage to be one piece in the process of casting the cylinder. The die can have a simpler shape and will be cheaper to produce.
If the engine is configured a valve controls the air flow supplied to the combustion chamber via the scavenging port of the engine. This air control valve is interlocked with the fuel-mixture control valve through a cam mechanism which rotates with the fuel-mixture valve. This allows the opening and closing of the air and fuel control valves to be controlled in relation to each other.
Thus when the engine is operating under a light load, the air control valve can be fully closed or opened only slightly so that the fuel mixture is richer and stable combustion can be maintained. When the engine is operating under a heavier load, the air control valve can be opened or closed proportionally with the fuel mixture control valve to produce a thinner mixture. We can thus provide an air-layer-type scavenging two-stroke engine in which the overall richness of the mixture can be kept at the appropriate concentration, the rate of fuel consumption is reduced, and the quantity of pollutants in the exhaust gas is lowered.
The opening of the mixture control valve which controls the flow rate of the fuel-air mixture is interlocked via a cam mechanism to the opening of the air control valve. The angular ratio of the mixture control valve to the air control valve can be set as desired; it will remain constant regardless of the angular position of the mixture control valve. This design, then, allows the user to select the most advantageous ratio.
(2) With the invention the air passage and the fuel mixture passage are linked by a small-diameter connecting passage. Thus the fuel mixture in the fuel passage can be supplied via this small-diameter connecting passage to the air passage. In this way more fuel-air mixture can be added to the air flowing through the air passage. This prevents the new air supplied to the cylinder from the scavenging port from creating too thin a mix when the engine accelerates suddenly, and thus improves the acceleration characteristics.
When the engine is operating at high speed, there is virtually no pressure differential between the fuel passage and the air passage. There will therefore be almost no fuel-air mixture flowing via the small-diameter connecting passage from the fuel passage to the air passage, and thus no fuel in the layer of preceding air. This will insure that the required exhaust specifications can be maintained.
When the engine is mounted obliquely, the fuel-air mixture in the fuel passage can flow into the air passage via the connecting passage. This prevents fuel from collecting in the lowest portion of the fuel passage so that a large quantity of fuel is suddenly sucked into the cylinder when the engine""s orientation is changed. This design, then, will prevent the imperfect combustion which would occur if there were excess fuel in the chamber.
Providing a small-diameter connecting passage between the air and fuel passages obviates the need for a control device with a complicated configuration. It allows us to realize an air-layer-type scavenging two-stroke engine which can achieve the same effect with an extremely simple device.
One end of the connecting passage connects the air and fuel passages downstream from the non-return valve. The effect of this when the engine is mounted obliquely is that it will prevent an excess of fuel in the combustion chamber which will result in imperfect combustion. The other end of the connecting passage connects the air and fuel passages upstream from the non-return valve. This makes it possible to increase the diameter of the outlet of the connecting passage without a drop in engine output, thus improving the engine""s ability to accelerate suddenly.
(3) With the invention a surface of a guide which forms the shape of a channel connects smoothly with the scavenger passage in the crankcase. The resulting scavenger passage is gradually curved with no right angles, so the fuel-air mixture which moves through it does not experience any loss of flow through deceleration, but is supplied to the scavenging port smoothly and at a high velocity. This improves the engine output.
The guide is mounted on the crankcase in such a way that it can be removed by pulling it away from the surface where the crankcase is attached to the cylinder along the axial direction of the cylinder. The guide fulfills the function of the projection that was used in the prior art. When the crankcase is cast, even if a single die is used to form the scavenger passage inside the crankcase, the die can easily be removed in the axial direction of the cylinder.
This design simplifies the casting procedure by which the scavenger passage is formed, and it reduces the number of casting processes required. Because the scavenger passage can be formed using a single die, there is no possibility that one of several dies will slip out of position, as sometimes happened with prior art techniques, and ruin the casting. This design, then, improves the quality of the crankcase containing the scavenger passage.
If the engine is configured the guide has a positioning tooth which engages with the hole in the gasket. When the tooth engages in the gasket hole, its position is guaranteed, and so is that of the gasket. The fact that the guide is in place can be known from how the gasket is seated, so there is no chance of forgetting the guide.
If the engine is configured the guide is fixed to the crankcase when its tooth engages in an indentation in that crankcase. In this way the guide can be positioned with complete accuracy so that its surface which forms a channel can connect smoothly to the scavenger passage in the crankcase.
If the engine is configured even if the fuel in the fuel-air mixture flowing through the scavenger passage connecting the crankcase and the cylinder through the common surface where they are attached to each other seeps into the gap between the surfaces where the gasket is inserted, the fuel will flow downward through the depression formed in the guide. Thus even if the engine is mounted obliquely, the fuel cannot return to the scavenger passage. This eliminates imperfect combustion due to fuel flowing back into the scavenger passage.
(4) If the engine is configured the blow-up angle of the upper wall of the scavenger passage which connects it to the scavenging port is greater at a location nearer the intake port than at one nearer the exhaust port. The fuel mixture which enters the chamber from the location nearer the exhaust port will flow along the top of the piston at a high speed without being dispersed. This will prevent it from getting caught in the exhaust gas stream and so reduce the quantity of fuel lost through the exhaust port. The fuel-air mixture which enters the chamber from the location nearer the intake port will be flowing at a lower velocity than that nearer the exhaust port. It will be sent into the area around the spark plug in the upper part of the chamber, where it will be efficiently ignited and combusted. The scavenging efficiency is improved, the fuel-air mixture which fills the combustion chamber has a higher concentration, and the combustion is improved, resulting in greater engine output. Also, preventing the fuel-air mixture from escaping reduces the level of THC (total hydrocarbons) in the exhaust.
If the engine is configured the surface of the upper wall of the scavenger passage is formed so that it varies in step fashion from a large blow-up angle at a location nearer the intake port to a smaller blow-up angle at a location nearer the exhaust port. When the cylinder is cast, two dies can be used with two different blow-up angles, with the angles changing at the border between the dies. This will make it easy to remove the work from the dies and will reduce the number of processes necessary to produce the cylinder.
Also, using dies with two different blow-up angles to form the scavenger passage is an easy and reliable way to control the blow-up angle.
(5) With the inventions disclosed in this application, the crankcase is divided into front and rear portions, each of which has a scavenger passage inside it. These two portions of the crankcase are fixed to each other by mounting hardware. The cylinder, whose scavenger passage connects to that in the crankcase, is fixed by mounting hardware to the mounting surface at the top of the crankcase. This results in a long scavenger passage running through both cylinder and crankcase. Both the crankcase and the cylinder are thus compact structures with no bulges, and the scavenger passage has a gradually curved contour without angularities.
Furthermore, a crankcase with a scavenger passage running through its interior can be cast in two pieces which form the front and rear portions of the engine. These can be removed from the die at the surface where the front and rear pieces are separated and at the surface where the cylinder is mounted to the crankcase, which is perpendicular to the surface between the pieces. This allows the die to have a simple shape, simplifies removing the work from the die, and allows the engine to be cast using fewer dies. The casting procedure is simplified and requires fewer processes, with the result that the cost is lower.
If the engine is configured as the scavenger passages will run the entire length from the outlets of the crankcase to the scavenging ports in the cylinder and through the surfaces where the two portions meet, and they will be symmetrical. Thus a common die can be used to cast the front and rear portions of the scavenger passages, allowing the engine to be produced with fewer dies. The shapes of the two passages will be identical, so the cylinder will be scavenged uniformly along its circumference and filled uniformly with the fuel-air mixture.
Furthermore, the two branching air passages run symmetrically through the surface between the two halves of the crankcase. If the passages are configured in this way, a common die can be used to cast both of them, so fewer dies need be used overall. Since the shapes of the two air passages are identical, the action of the scavenging air will be uniform along the circumference of the cylinder.
Also, the walls of the scavenging and branching air passages can be formed integrally to the cylinder and virtually parallel with each other. If configured in this way, the sliding die for the cylinder can be a single piece. This simplifies the configuration of the die and reduces the cost of producing it.
(6) With the invention the first air passage from the air cleaner is completely and reliably closed by the choke valve when the engine is being started up. Thus the supply of scavenging air to the combustion chamber is cut off, and only the fuel-air mixture from the carburetor is supplied to the combustion chamber. Thus the chamber is filled with a rich fuel-air mixture. This improves the start-up characteristics of the engine.
The flat surface of the rotary choke of the choke valve covers and uncovers the openings of the two air passages. The elastic force of the O-ring or other sealing ring which is inserted around the shaft of the choke causes the flat surface to press against the openings. If configured in this way, the choke valve completely and reliably closes the opening of the passage for the preceding air. This promotes the production of a rich fuel-air mixture as described above and allows a high negative pressure to be maintained.
In addition to the sealing ring, a protrusion is provided on the outside of the case of the air cleaner to serve as a stop for the choke. If the choke valve is configured in this way, rotating it from its initial position to its normal operating position will switch between the two aforesaid air passages. When the choke valve is rotated, its knob easily goes over the protrusion against the force of the sealing ring. The moderate friction improves the operating feel of the choke valve. When the user releases the choke valve after operating it, the elastic force of the sealing ring and the force of the protruding stop automatically hold the choke valve in place on the flat portion of the case in such a way that it cannot go back. This insures easy operation.
Furthermore, there is a tapered protrusion on the inner surface of the case of the air cleaner. When the choke valve is rotated toward starting position, the knob of the choke valve goes over the protrusion. When this occurs, the elastic force generated by the deflection of the valve causes its flat surface to push down on the opening of the passage for the preceding air. This improves the sealing function of the flat surface, and thus improves the start-up characteristics.